1. Field of the Invention
The present invention relates to a low dielectric constant insulating film and a method for forming the same. In particular, the present invention relates to an insulating film having a low dielectric constant as well as a high mechanical strength and a high damage resistance, and a method for forming the same.
2. Description of the Related Art
With increase in integration degree and operation speed of semiconductor devices, it is demanded to drop a capacity between wirings. In order to drop the capacity between wirings, it is necessary to develop a technique of lowering dielectric constant of an interlayer insulating film. For that reason, an insulating film having a dielectric constant of 3.0 or less, so-called low-k film has been studied.
Conventionally, the low-k film has been produced by a plasma CVD method. In the plasma CVD method, a substrate is placed on a stage in a chamber, a raw gas containing for example, CH3 group is introduced into the chamber, and the raw gas is ionized to produce plasma and is polymerized, thus depositing a low-k film on the substrate (see, for example, Shin-Ichi Nakao et al. “UV/EB Cure Mechanism for Porous PECVD/SOD Low-k SiCOH Materials”, IITC, 2006 IEEE, p. 66-68, Y. Hayashi et al. “Novel Molecular-structure Design for PECVD Porous SiOCH Filmes toward 45 nm-node, ASICs with k=2.3”, IITC, 2004 IEEE, p. 225-227, N. Tajima et al. “First-principle Molecular Model of PECVD SiOCH Film for the Mechanical and Dielectric Property Investigation”, IITC, 2005 IEEE, p. 66-68).
In the conventional method of producing the low-k film using the plasma CVD method, however, precursor molecules contained in the raw gas are dissociated more than necessary by high energy electrons, ultraviolet rays, or photons emitted from plasma. For example, CH3 group is eliminated from Si—CH bond in the precursor molecules to an excessive degree by excessive energy of electrons, ultraviolet rays, or photons. Similarly, organic group is eliminated from the low-k film deposited on the substrate. Thus, where dissociation of gases is promoted by means of generation of plasma, it is impossible to form a CVD film having a desired molecular structure. It is therefore difficult to form a film having a desired dielectric constant (k<2.2) and high strength (Young's modulus of elasticity 4.0).
Further, in order to drop a dielectric constant of the film, various methods have been employed. Those methods include incorporating many organic groups into the film to increase steric hindrance structure and to lower the film density, and adding porogen to the film and burning it to form pores in the film and to lower the film density. However, assuming that k value of the raw gas molecule is 3.0, a porosity of about 50% is necessary in order to form an insulation film having k value of 2.0 or less. In the method of adding porogen, therefore, there is a relation of a trade-off between dielectric constant and mechanical strength.
For example, a molecular pore stacking (MPS) film having a low dielectric constant induced by forming pores of molecule size in the film, is proposed. However, thus formed MPS film has k value of only about 2.4. In order to lower dielectric constant of the MPS film, it is necessary to expand a diameter of a SiO ring. In this case, however, strength of the film drops.
In order to lower dielectric constant with keeping strength of the film, it is proposed to introduce SiO2 content into the raw molecules. This technique, however, lowers a concentration of CH3 in the film by increasing SiO2 content in the film to deteriorate resistance to plasma damage. In general, it is said that the film having a low dielectric constant and high strength has a low resistance to plasma damage.
On the other hand, as the technique of reflecting the structures of raw gas molecules in a deposited film without destroying the structures of raw gas molecules, NBE-CVD method is proposed. In the NBE-CVD method, the raw gas molecules adsorbed onto the surface of the substrate are irradiated with neutral beams to polymerize the raw gas molecules and deposit an insulating film on the substrate.
In the NBE-CVD method, since the raw gas and the deposited film are not irradiated with ions and UV light unlike the conventional plasma CVD method, it is possible to realize irradiation of a very low energy and to synthesize a film material without destroying the structures of the raw gas molecules. Particularly, in SiO—OCH3 based gas having a methoxy group, since bonding energy of between O atom and CH3 group is weak, it is possible to synthesize a low-k film of SiOCH3 series by cutting the bond between O atom and CH3 group. According to this technique, since the irradiation energy is relatively low and Si—CH3 bond are hardly cut, it is possible to incorporate CH3 of a high concentration in the film.
The NBE-CVD method, however, has a drawback that the film forming speed is low since it utilizes an adsorption phenomenon of the raw gas molecules onto the substrate. In this case, since the adsorption probability is improved by increasing molecular weight of the raw gas, it is possible to improve the film forming speed. However, when irradiation of neutral particle beams is continuously performed, a temperature of the substrate rises to lower the adsorption probability of the raw gas molecules, and a film of uniform quality is not obtained.